This invention relates generally to gas turbine engines and, more particularly, to an apparatus for minimizing rotor/shroud clearance during both steady-state and transient operation.
As turbine engines continue to become more reliable and efficient by changes in methods, designs and materials, losses which occur from excessive clearances between relatively rotating parts become more important in the many design considerations. In many turbine engine applications, there is a requirement to operate at variable steady-state speeds and to transit between these speeds as desired in the regular course of operation. For example, in a jet engine of the type used to power aircraft, it is necessary that the operator be able to transit to a desired speed whenever he chooses. The resulting temperature and rotor speed changes bring about attendant relative growth between the rotor and the surrounding shroud and, in order to maintain the desired efficiency, this relative growth must be accommodated. The primary concern is in maintaining a minimum clearance between the stator and rotor while preventing any frictional interference therebetween which would cause rubbing and resultant increase in radial clearance during subsequent operation. When considering the transient operating requirements as mentioned hereinabove, the relative mechanical and thermal growth patterns between the rotor and the shroud present a very difficult problem.
Various schemes have been devised to variably position the stationary shroud in response to engine operating parameters in order to reduce rotor/shroud clearance. One such scheme is that of the thermal actuated valve as described in U.S. Pat. No. 3,966,354, which is assigned to the assignee of the present invention. In that apparatus, a valve operates in response to the temperature of the cooling air and, to the extent that the cooling air temperature is dependent on the speed of the engine, the transient condition is considered. However, such a system tends to be relatively slow in responding and relatively inaccurate in trying to match relative growth during transient operation.
Probably the primary reason that a cooling air system operating only on a speed responsive schedule is inadequate is that such a system is not capable of taking into account the thermal heating and cooling time constants of the rotor for all possible sequences of transitional operation. That is, present systems are only capable of matching thermal time constants of the rotor when the sequence of transient condition operation is known. This, of course, is not acceptable since the particular operating mode and sequence of operation is going to depend on the mission at hand.
It is therefore an object of the present invention to provide an efficient turbine engine which is capable of transiting between various speeds while maintaining a minimum clearance between its rotor and shroud.
Another object of the present invention is the provision in a turbine engine for responsively modulating the position of a rotor shroud in response to multiple steady-state and transient operation conditions.
Yet another object of the present invention is the provision in a clearance control system for selectively varying the shroud position in response to the thermal time constants of the rotor.
Still another object of the present invention is the provision for a rotor/shroud clearance control system which is responsive and effective over a wide range of steady-state and transient operating conditions.
Yet another object of the present invention is the provision for a rotor/shroud clearance control system which is economical to manufacture and relatively simple in operation.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.
Briefly, in accordance with one aspect of the invention a timing valve acts in response to rotor speed signals to schedule temperature changes to the flow of air to the turbine shroud support so as to match the thermal time constants of the rotor. In this way, the clearance between the rotor and shroud can be minimized during both transient and steady-state operation.
By another aspect of the invention the timing valve is activated upon the reaching of a predetermined level of rotor speed. It then advances at a constant rate to incrementally schedule increases in the temperature of the air.
By another aspect of the invention the air temperatures are varied by the use of two air sources at different temperatures. They are selectively used independently or mixed so as to obtain air at four different modes of airflow.
By still another aspect of the invention the timing valve, upon the rotors decreasing to a predetermined speed level, begins to retract at a constant speed toward its original position. The speed of retraction is slower than the speed of advance so as to accommodate rebursts without rotor-to-shroud interference. During the retraction phase, the air delivery is determined by rotor speed and is independent of timing valve position.
By another aspect of the invention the timing valve continues to advance for a predetermined time after reaching the highest temperature mode of airflow so that during the retraction period, the resulting additional time allows the rotor to cool sufficiently to permit rotor bursts without attendant rub.
By yet another object of this invention the scheduling function of the timing valve is pre-empted by the rotor's operation at predetermined speeds. The temperature of the air delivered is then determined solely by the rotor speed speed and irrespective of the position of the timer valve.
In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.